This invention relates generally to a novel method for molding or curing a resin material at a temperature of at least about 120xc2x0 F. (about 50xc2x0 C.), and more particularly at least about 350xc2x0 F. (about 180xc2x0 C.), using a multilayer release film as a barrier at the surface of the resin material. This invention also generally relates to a novel release film which may be used with such a method.
There is a significant need in a variety of industries for producing strong, durable, light weight structural parts. This is particularly true in the automotive, aircraft, and aerospace industries, where these parts play an important role in producing energy efficient vehicles, airplanes, and spacecraft. In response to this need, many manufacturers have turned to developing resin materials which match or exceed the strength and durability of the traditionally used metal parts.
Release films (also known as xe2x80x9cparting filmsxe2x80x9d or xe2x80x9cbarrier filmsxe2x80x9d) play an important role in molding and curing processes of the parts made from resin materials. Release films, for example, are used to separate a mold (or xe2x80x9ctoolxe2x80x9d) from the resin material so that the material does not stick to the mold. Such molding and curing processes often are performed at temperatures of about 350xc2x0 F. (about 180xc2x0 C.) or greater, and, therefore, require the release film to be heat stable at such temperatures.
In many traditional applications, the release films used in the curing and molding processes of resin materials have been monolayer films prepared from various commercially available polymeric thermoplastic resins, such as fluoropolymer resins. These monolayer films, however, generally exhibit less-than-satisfactory physical characteristics (e.g., tensile strength, tear strength, and elongation) and/or tend to be costly. In some instances, inexpensive monolayer films having unsuitable release characteristics (but otherwise desirable physical characteristics) have been coated with a release agent to impart the desired release characteristics to the film. Traditionally used release agents include, for example, siloxanes, silicones, diethylene glycol monostearate, hydrogenated castor oil, stearic acid, oleic acid, zinc stearate, calcium stearate ethylene bis(stearamide), oleyl palmitamide, microcrystalline wax, paraffin wax, carnauba wax, spermaceti wax, cellophane, cellulose acetate, sodium alginate, and combinations thereof. Use of such release agents, however, is often undesirable because the release agents tend to create a risk of contamination to the resin material being cured or molded. It also has been reported that some release agents can create a health hazard. See Higgins, U.S. Pat. No. 5,932,352.
In response to the deficiencies of monolayer release films, multilayer release films have begun to be developed. Many of these films reportedly tend to be cheaper and exhibit better physical characteristics than the traditional monolayer films.
Multilayer release films, for example, have reportedly been particularly useful in molding and curing processes of sheet molding compounds. See, e.g., Maligie, U.S. Pat. No. 5,601,770. Such processes, however, are generally performed at temperatures of from 175 to 340xc2x0 F. (80 to 170xc2x0 C.). See, e.g., Akiyama, et al., U.S. Pat. No. 5,783,635 (176 to 320xc2x0 F.; 80 to 160xc2x0 C.); Takabatake, et al., U.S. Pat. No. 5,834,547 (248 to 302xc2x0 F.; 120 to 150xc2x0 C.); Butler, U.S. Pat. No. 5,744,816 (176 to 212xc2x0 F.; 80 to 100xc2x0 C.); Hesse, et al., U.S. Pat. No. 5,414,043 (248 to 338xc2x0 F.; 120 to 170xc2x0 C.). These temperatures are less than the temperatures used in many other resin molding and curing processes, and, therefore, have less stringent heat-stability requirements for the release films than do the higher temperature processes.
In U.S. Pat. No. 5,080,979, Shigemoto et al. disclose multilayer release films having a polyolefin layer sandwiched between two polymethylpentene layers. Although the specific films disclosed by Shigemoto et al. may be used in some resin curing and molding processes, it has been reported that such films lose their dimensional stability at temperatures of about 350xc2x0 F. (about 180xc2x0 C.) and greater. See Tsai et al., U.S. Pat. No. 5,858,550. Thus, they generally must be used at lower temperatures, therefore necessitating longer periods of time to cure or mold the resin materials.
One approach which reportedly addresses the problems associated with Shigemoto et al.""s release films is disclosed by Tsai et al. in U.S. Pat. No. 5,858,550. Tsai et al. disclose that certain multilayer films (e.g., a 5-layer film having two exterior layers of polymethylpentene, a core layer of nylon 6, and an adhesive layer of maleic anhydride modified polypropylene between the core layer and each exterior layer) may be used as release films when preparing printed circuit boards at temperatures of at least about 350xc2x0 F. (about 180xc2x0 C.) if the films are first uniaxially or biaxially oriented (i.e., stretched in a certain direction(s)). In particular, Tsai et al. disclose using either uniaxially or biaxially oriented films, especially uniaxially oriented films produced at stretch ratios of from 2:1 to 6:1. Tsai et al. define this xe2x80x9cstretch ratioxe2x80x9d as the increase of dimension in the direction of the stretch (e.g., a film having a stretch ratio of 2:1 has its length doubled during the stretching process). According to Tsai et al., such stretching may be accomplished by passing the film over a series of preheating and heating rolls. In such an apparatus, the heated film moves through a set of nip rolls downstream at a faster rate than the film entering the nip rolls at an upstream location. The change of rate is compensated for by stretching in the film. Although Tsai et al. report that this stretching step imparts certain characteristics to the release film that are necessary for the film to function properly at temperatures of at least about 350xc2x0 F. (about 180xc2x0 C.), inclusion of such a step adds to the costs associated with manufacturing the release film.
Among the objects of this invention is the provision of a novel method for curing or molding resin materials which employs a release film that has one or more of the following characteristics: (1) it may be used with typical resin molding and curing processes at temperatures of at least about 350xc2x0 F. (about 180xc2x0 C.); (2) it is economical to manufacture, and preferably less costly to prepare than the release films that traditionally have been used to mold and cure resin materials at temperatures of at least about 350xc2x0 F. (about 180xc2x0 C.); and (3) it may be used without having a release agent coated on its surface, thereby avoiding the risk of contamination by the release agent.
Briefly, therefore, this invention is directed to a method for molding or curing a resin material using a multilayer release film as a barrier at the surface of the resin material. In one embodiment, the method comprises the surface of the resin material being in contact with the release film at a temperature of at least about 350xc2x0 F. (about 180xc2x0 C.). Here, the release film is non-oriented and comprises: (1) an enhancement layer which comprises a thermoplastic material; and (2) a first release layer which has a composition that is different from the enhancement layer, comprises a polymeric material, and is releasable from the resin material after the resin material is molded or cured.
In another embodiment directed to a method for molding or curing a resin material using a multilayer release film as a barrier at the surface of the resin material, the method again comprises the surface of the resin material being in contact with the release film at a temperature of at least about 350xc2x0 F. (about 180xc2x0 C.). Here, the release film comprises (1) a release layer which comprises polymethylpentene; (2) an enhancement layer which comprises polyamide; and (3) a tie layer which is sandwiched between the release layer and enhancement layer and comprises (i) polyethylene or modified polyethylene, and (ii) polypropylene or modified polypropylene.
In yet another embodiment directed to a method for molding or curing a resin material using a multilayer release film as a barrier at the surface of the resin material, the surface of the resin material is contacted with a release film comprising 5 layers in the following order (1) a first release layer which comprises polymethylpentene; (2) a first tie layer which comprises (i) polyethylene or modified polyethylene, and (ii) polypropylene or modified polypropylene; (3) an enhancement layer which comprises polyamide; (4) a second tie layer which comprises (i) polyethylene or modified polyethylene, and (ii) polypropylene or modified polypropylene; and (5) a second release layer which comprises polymethylpentene.
In a further embodiment directed to a method for molding or curing a resin material using a multilayer release film as a barrier at the surface of the resin material, the surface of the resin material is contacted with a release film comprising: (1) a release layer which comprises polymethylpentene; (2) an enhancement layer which comprises polyamide; and (3) a tie layer which is sandwiched between the release layer and enhancement layer and comprises anhydride modified linear low density polyethylene and anhydride modified polypropylene.
Another object of this invention is the provision of a release film that may be used, for example, with the above-described method.
Briefly, in one embodiment, this invention is directed to a release film comprising 5 layers in the following order: (1) a first release layer which comprises polymethylpentene; (2) a first tie layer which comprises (i) polyethylene or modified polyethylene, and (ii) polypropylene or modified polypropylene; (3) an enhancement layer which comprises polyamide; (4) a second tie layer which comprises (i) polyethylene or modified polyethylene, and (ii) polypropylene or modified polypropylene; and (5) a second release layer which comprises polymethylpentene.
In another embodiment directed to a release film, the release film comprises: (1) a release layer which comprises polymethylpentene; (2) an enhancement layer which comprises polyamide; and (3) a tie layer which is sandwiched between the release layer and enhancement layer and comprises anhydride modified linear low density polyethylene and anhydride modified polypropylene.